It has been known for many years that porous valve-metal capacitors employing the valve metal as one capacitor plate, the anodic oxide as the dielectric, and a solid-state electrolyte, such as manganese dioxide, as the second capacitor plate (typified by so-called "solid" tantalum capacitors) give more reliable service with respect to device short-circuit prevention when the field through the anodic oxide is relatively low, i.e. when the voltage applied to the device in service is significantly lower than that used to produce the anodic oxide dielectric film. As a consequence of this fact, manufacturers employ anodizing voltages that are 2 to 5 (or more) times as high as the service-rated voltage of the devices.
It has also been known for many years that the presence of a thicker anodic oxide film on the external surfaces of valve-metal anode bodies offers a good deal of protection from short-circuiting of the devices when the anodic oxide film thickness on the internal surfaces of the anode bodies is minimized for the purposes of minimizing the use of expensive anode materials, such as tantalum, or for maximizing the device capacitance for a given anode size. There are at least 3 contributing factors which combine to make thickness of the external oxide of greater importance with respect to device reliability than the interior oxide thickness:
1) The radius of curvature of the corners and edges of the anodes raises the electric field in these areas, making them more prone to short-circuit failure.
2) The voltage drop across the "solid" electrolyte (e.g. manganese oxide) is lowest on the outer surface of the anodes, making somewhat higher currents available to oxide flaws on the external anode surfaces.
3) The external anode surfaces, particularly corners and edges, of powder-metallurgy porous valve-metal anode bodies become damaged during bulk handling in the manufacturing process. Anodes abrade against each other, as well as against feeder bowl and vibratory track surfaces, etc. The abrasion encountered during bulk handling has been demonstrated to cause the passive oxide film (formed upon contact with the atmosphere after the sintering operation) to become imbedded in the external anode surfaces. This abrasively-imbedded passive film gives rise to flaws in the anodic dielectric film on abraded areas of the anode (described in U.S. Pat. No. 5,716,511). Although oxide "blisters" become readily apparent at anodizing voltages above about 100 volts, the incipient flaws are present at all film thicknesses and give rise to disproportionately high leakage currents with increasing field.
Several techniques for producing a thicker anodic oxide film on the external surfaces of anodized porous anode bodies are described in the patent literature.
U.S. Pat. No. 3,415,722 (Scheller, et al.) describes the method of anodizing porous capacitor bodies to form a uniform dielectric film throughout said anode bodies, impregnating the anode bodies with a solid, non-conducting material (wax, stearin, anthracene, etc.) removing the insulating solid from the external anode surfaces and anodizing at a voltage higher than the initial voltage of the external surfaces to form an oxide film on the external surfaces which is thicker than the internal anodic oxide. The insulating solid is then removed from the bodies of the anodes prior to processing into finished capacitors.
A somewhat similar process is described in U.S. Pat. No. 5,643,432 (Qiu). This patent describes the production of a thicker external oxide via the impregnation of an anodized porous body electrode with an electrolyte-insoluble, insulating liquid (benzene, xylene, etc.), evaporation of the insulating liquid from the external surfaces of the anode body, and formation of a relatively thick anodic oxide layer on the external surfaces of the anode bodies at a voltage higher than the initial anodizing voltage. The remainder of the solvent is evaporated prior to processing the anodes into finished capacitors.
A differential anodizing process (i.e., an anodizing process for porous valve-metal capacitor bodies producing a uniform internal anodic oxide thickness with a significantly thicker oxide on the external anode surface) is described in U.S. Pat. No. 4,131,520 (Bernard, et al.). This patent describes the production of a thicker external anodic oxide film on anodized porous anode bodies via the use of electrolytes consisting of aqueous solutions of the salts of weak acids (e.g. borax 0.01%-4.0% or ammonium pentaborate/boric acid). The flow of electric current gives rise to a partitioning of the electrolyte into an anion-rich portion inside the anodes and a cation-rich portion external to the anode bodies. The anion-rich electrolyte inside the anode bodies tends to undergo re-association to form the weak acid, giving a relatively high electrolyte resistivity inside the anodes and an associated much higher rate of anodic oxide formation on the external anode surfaces. Again, a higher anodizing voltage is used to produce the external anodic oxide film.
U.S. Pat. No. 4,131,520 also relates an additional method of achieving differential anodizing; the anodized porous valve metal anodes are dipped into a solvent which does not readily transport the anions of weak acid salts into the bodies of the porous anodes and the solvent-dipped anodes are then anodized at a voltage higher than the initial anodizing voltage to produce the thicker external anodic oxide.
More recently, another differential anodizing method is described in U.S. Pat. No. 5,837,121 (Kinard et at.), in which the non-thickness-limited anodizing capability of solutions of dibasic potassium phosphate in glycerine containing less than 0.1% water and operated above about 150.degree. C. is employed to produce a relatively thick layer of anodic oxide on the external surfaces of anodes while producing relatively little oxide growth on the internal surfaces of the anode bodies (The non-thickness-limited anodizing process is also described in detail in the paper entitled: "The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals", by Brian Melody, Tony Kinard, and Philip Lessner, which was published in Electrochemical and Solid State Letters, Vol. 1, No. 3, pages 126-129 , 1998).
All of the above methods of differential anodizing (i.e., anodizing which produces a thicker anodic oxide film on the external surfaces than on the internal surfaces of porous, value-metal anode bodies) have serious drawbacks in commercial capacitor production. The method described in U.S. Pat. No. 3,415,722 produces anodized anodes containing a waxy substance which is difficult to remove completely and which may interfere with the solid electrolyte impregnation process if incompletely removed.
The process described in U.S. Pat. No. 5,643,432 gives rise to the leaching of water insoluble liquid organic materials from the anodes during the second anodizing step. This gives rise to uncertain shielding of the internal portions of the anodes so-treated from the second anodization step electrolyte, resulting in unpredictable capacitance loss and electrolyte contamination.
The first process described in U.S. Pat. No. 4,131,520, that of employing an aqueous solution of the salt of a weak acid, is very sensitive to the presence of contamination; specifically, any phosphate, etc., remaining within the anode bodies from the initial anodizing step tends to defeat the differential nature of the anodizing process, resulting in the production of additional oxide film growth on the internal surfaces of the anode bodies with the loss of device capacitance. The second process described in U.S. Pat. No. 4,131,520, that of dipping the anodes in de-ionized water or ethylene glycol prior to the second anodization step, requires the additional liquid-dipping step and a short duration between the liquid dipping step and the second anodization step to prevent erratic results due to solvent evaporation. Also, when ethylene glycol or other organic material is used, the anodizing electrolyte becomes progressively contaminated with the organic solvent.
The method described in U.S. Pat. No. 5,837,121, which makes use of non-thickness-limited anodizing phenomena to achieve differential anodizing, requires the use of electrolyte at a temperature above about 150.degree. C., which is difficult to maintain and manipulate in production equipment which is not specifically designed for this purpose (most anodizing tanks are designed for use below 100.degree. C.).
All of the above methods of differentially anodizing anodes so as to produce a thicker external oxide, with the exception of the process described in U.S. Pat. No. 5,837,121, call for the anodes to first be anodized to form uniformly thick anodic oxide throughout the porous anode bodies prior to the anodization step used to produce the thicker external oxide. The electrolytes used to anodize porous anode bodies almost always contain the orthophosphate ion due to the enhanced thermal stability of anodic oxides formed in phosphate-containing electrolytes. Unfortunately, the same incorporated phosphate which enhances the thermal stability of the anodic oxide, also tends to reduce the wettability of the oxide by aqueous solutions, including the electrolytes used to produce the thicker external oxide film during the secondary anodization process. The reduced wettability of the anode surfaces covered with phosphate-doped anodic oxide results in the non-uniform production of the thicker external oxide due to the presence of air/gas bubbles present in contact with the external surface of the porous anode bodies and not displaced by the electrolyte used to produce the thicker external anodic oxide film This problem is addressed by the anodizing method given in U.S. Pat. No. 4,278,513, in which 0.01% to 1.0% of a non-ionic surfactant is added to the second-stage electrolyte in order to affect anode-wetting and to displace any air/gas bubbles in contact with the anodes.
Additionally, there is evidence that the anodic oxide stoichiometry which is largely established during the constant voltage portion of the first-stage anodizing step (which produces the oxide of uniform thickness throughout the anode bodies) is upset by the rapid growth kinetics in the second-stage anodizing step which produces the thicker oxide on the external anode surfaces.